Designer 2.1 does passively "upgrade" the project to DmT 2.1. If you save it to disk with 2.1, it will not be openable with 2.0.3. If you try to write it to a PLC, it will not download unless the firmware is upgraded to DmT 2.1.
So, if you do NOT upgrade your firmware at this time, you need to open your DmT 2.0 projects using 2.0.3.
Possible alternative scenario
You could open the 2.0 Project in 2.1, do a Save-As to your 2.1 Projects folder, and do some offline work in 2.1. So you would still have your DmT 2.0 project in your 2.0.3 Projects folder, your PLC would still be running DmT 2.0 with the 2.0.3 Project, but you would have an offline DmT 2.1 project "waiting in the wings" in your 2.1 Projects folder whenever you wanted to download it to a PLC with upgraded firmware with DmT 2.1 firmware.